Soundproof wall



March 26, 1940. E. MEYER SOUNDPROOF WALL Filed llay 20, 1957 /n venzol:15/2 m/v J1 Y R Patented Mar. 26, 1940 UNITED STATES PATENT OFFICEApplication May 20, 1937, Serial No. 143,837 In Germany July 27, 1935 3Claims.

An application was filed in Germany July 2'7, 1935.

This invention relates to the subject of sound absorption and moreparticularly to the con- 6 struction of sound-proof walls for buildings,

ships and airplanes. t

A primary object of the invention is to provide sound-proof wallsconstructed in accordance with a given formula certain factors of whichmay be varied as the circumstances require. In that connection theinvention contemplates the sound-proofing of walls by absorbing soundfrequencies within a calculated range by analogy to the control ofelectrical frequencies used in space transmission.

The invention is illustrated in the accompanying drawing, in which:

Figures 1-4, inclusive, are diagrammatic sectional views illustratingthe several phases of the invention.

Similar reference characters designate corresponding parts throughoutthe several views.

Before proceeding to a description to a theory of the invention it maybe pointed out that Figure 1 shows a double wall wherein sound absorbingmaterials are provided in the hollow space adjacent the edges.

Figure 2 shows a double wall unit wherein the sound absorbing materialsare arranged in the middle of the space formed between the wall.

. Figure 3 illustrates a double wall wherein the interior hollow spaceis sub-divided by transverse walls or baflles.

Figure 4 illustrates a modification of the in- 85 vention wherein threespaced walls are utilized together with sound absorbing materialsinteriorly arranged along the edges in accordance with the teaching ofFig. 1.

In each of the figures above referred to the masonry or iron beams orgirders of the building is indicated as I, while 2 designates walls orshells consisting of masonry, concrete or any suitable buildingmaterial. The sound absorbing material such as glass wool, slag wool,felt or the like is designated as 3. 1

In Figure 3 where interior transverse walls or baflies 4 are used thepockets or cells 5 are formed. The members 4 may be fiber plates, metalplates or the like.

I In all of the figures the character 1 indicates the width or depth ofthe air gap.

It is known and has been proved by numerous scientific investigationsthat the sound transmission of a simple wall substantially depends I8upon the mass of the latter. According to this knowledge, considerablesound insulation is only attained by heavy walls. Furthermore, it isknown that multiple walls, that is to say, walls consisting of aplurality of individual shells with air spacing aiford somewhat morefavourable conditions. Heretofore, no particulars have been availableregarding their optimum dimensions and also regarding the nature of theair gap. This knowledge gained from numerous tests forms the basis ofthe present invention. Information regarding the optimum dimensioning ofmultiple walls may be derived from the similarity with electrical filtercircuits. The individual shells act like masses and the air-gaps likesprings, that is to say, the individual shells correspond electricallyto the inductances and the air gaps to the capacities. Low pass filterstransmit low-frequency alternating currents without any considerableattenuation, while on the contrary they attenuate very considerablyalternating currents having a frequency beyond a certain frequency, theso-called cut-off frequency. If m signifies the mass of the wall persquare centimeter and I the magnitude of the air gap (dis tance betweentwo walls), then applying this knowledge according to the invention forsoundinsulating walls, the value is obtained for the cut-off frequencyin cycles per second'for sound vibrations passing through the wall.Applicant is the first to disclose this formula, whose validity he hasestablished by comprehensive series of experiments. The form of thisequation is suggested by the analogy of known relations of frequency,inductance, and capacity in an electrical low pass filter, butexperimental work was necessary to establish its validity in acousticphenomena in walls. In this formula p signifies the air density in gramsper cubic centimeter and c the velocity of sound in centimeters persecond. The cut-off frequency is therefore lower, the greater the massof an individual shell and the greater the air spacing. The soundabsorption or the impenetrability to sound of a wall must be of equalmagnitude for all occurring pitches of sound. For the multiple wall, thedimensioning rule may therefore be expressed by stating that theindividual shell mass and the air spacing should be selected so that thecut-off frequency is lower than the practically occurring and disturbingpitches of sound. For example, a cut-off frequency of 100 cycles shouldsufllce for many purposes; in other cases in which the source ofdisturbance is of a higher frequency, a higher cut-off frequency initself will suffice. Its position also depends upon whether the soundprocess on the other side of the wall cannot be heard at all or, likespeech, can merely no longer be understood. Experiments have shown thatfor a simple wall with two shells, for the most widely differingcombinations of wall shell mass and air gap magnitude, the value givenby the above formula for the cut off frequency agrees excellently withobserved values. On the other hand, in a multiple wall consisting ofthree or more shells and a series of wall masses and air cushions, asubstantially smaller attenuation is observed than would be expectedaccording to formulas based on the electrical analogy.

The multiple wall has a salient cut off frequency which is imparted toit by the mass of the wall and the length of the air cushion. Thetransmission velocity through the multiple wall for particularindividual frequencies is found to decrease as the pitch of theimpinging sound approaches the cut-off frequency. While the velocity ofsound may be considered to be the same for all frequencies for ahomogeneous wall, this condition does not obtain for a non-homogeneouslaminated wall consisting of stacked layers of different media. Theserelations also are along the lines of the analogous electrical filter.

The smaller value of attenuation for the multiple wall particularly inthe neighborhood of the cut-01f frequency, than would be expected fromthe electrical analogy, may be explained by considering the fact thatwhile the distance between adjacent shells of the multiple wall,considered up to even high frequencies, is generally less than the wavelength, but on the other hand the dimension of the air cushion parallelto the surface of the shells is on the contrary not small with referenceto the wave length of the impinging sound. It has been found thattransverse vibrations or oscillations and hence resonance effects arecaused in these air cushions parallel to the surface of the shells, andthese oscillations diminish the sound insulation.

The second feature of the present invention, therefore, resides ininterrupting or preventing such oscillations and resonance elfects byinterrupting the spread of sound in directions parallel to the shellsurfaces, or by using sound absorbing materials in the air chamber. Inthis manner, these oscillations and resonance effects can be eliminated,and attenuation or damping effects can be produced which rise sharplywith the frequency. For example, it has been found that a multiple wallbuilt along the lines here described and having a wall weight ofkilograms per square meter, has a transmission loss of decibels, thatis, it corresponds to a solid brick wall 50 centimeters thick andweighing 1000 kilograms per square meter. This interruption may beeffected in various ways, for example by the insertion of light orporous materials, or by subdividing the air cushion by means of latticework in such a manner that the newly formed spaces, in respect of theirdimensions parallel to the wall surface, are small in comparison withthe wave length. In some cases, it is merely necessary, according to theinvention, to apply the porous sound-absorbing substances or materialsonly at the edges of the air cushion. The resonance of the transverseoscillations of the air cushion is likewise prevented thereby, and hencethe sound absorption is considerably increased.

With the said means, the sound attenuation is increased particularlybeyond the cut-oil frequency, that is to say from the cut-off frequencytowards the higher tones. In order also to diminish the transmission ofsound below the cutoff frequency-this being the third feature of the newwall combination-it is necessary, in the same way as in the case ofelectric filters, to introduce a reflection damping factor in thetransmission between the individual elements of the multiple wall. Thisis effected as follows: The cut-off frequency is a function of theproduct of the wall mass and depth or length of the air cushion and morespecifically varies inversely with the square root of the product ofwall mass and depth of air cushion, in accordance with the formula abovegiven. The acoustic impedance of the system is a function of thequotient; of unit wall mass m divided by the depth 1 of the spacebetween adjacent shells or walls, and is given by the equation where pis the density of air in grams per cubic centimeter and c is the soundvelocity in centimeters per second. This formula has been empiricallyderived by applicant by a study of his own extended experimentalobservations, and its form is suggested by the analogy of thecorresponding electrical filter relations. For the purposes of thepresent application, the essential feature is that the acousticimpedance is a function of the quotient of unit mass divided by airspace depth, while the cut-off frequency is a function of the product ofthese same quantities. In order to'diminish the transmission of sound inthe frequency range of transmission, the above-mentioned quotient isselectively varied in such manner as appears desirable for the severalindividual sections of the multiple well, while the product of thesequantities of wall mass and air cushion depth is kept substantiallyconstant. Additional attenuation is thereby introduced between theindividual members of the multiple wall. The sound is reflected at thepoint concerned, whereby the sound attenuation is increased. Of coursethe last means is only possible with walls having three or more shells,while the earlier consideration relates also to walls with two shells.The materials from which the wall is made are without significance, themagnitude of their mass alone being important. Practical tests haveshown that by applying the means described herein, walls may be madehaving a sound transmission less than that of a solid brick wall of thesame thickness and 20 times heavier. A necessary condition for securinga high transmission loss to sound is of course the prevention of solidsound bridges between the individual walls. If solid connections betweenthe walls are necessary for static reasons, solid sound-absorbingmaterials (cork, felt, rubber and so forth) or springs or both should beinserted at those places for isolating the sound.

I claim:

1. In a. multiple-layer sound-proof wall to transmit substantially nosound vibrations of frequencies exceeding a predetermined cut-01ffrequency 12 cycles per second, a plurality of substantially parallelshells respectively of different masses per square centimeter of shellsurface, m1, ma, ma, by air gaps of different lengths in centimeters,Z1, l2, l3, said pairs of values of mass and and respectively spacedapart' length being so selected that the products mili, mzlz, malaremain substantially equal to each other while the quotients l1 1 127 lavary over a considerable range, said values of length, l1, 12, 13 beingrespectively determined for the corresponding masses m1, m2, ma, by theform of equation ar n m I where c is the velocity of sound in air incentimeters per second and p is the density of air in grams per cubiccentimeter.

2. A wall as set forth in claim 1, including a lattice structure in eachof said gaps formed of interlaced strips substantially perpendicular tothe faces of said shells.

3. A wall as set forth in claim 1, including a lattice structure in eachof said gaps formed of interlaced strips substantially perpendicular tothe faces of said shells and separated from each other by spaces whichare small in comparison with the cut-off wave length.

ERWIN MEYER.

